As methods for joining molded articles made of thermoplastic resins together, welding techniques have conventionally been known including hot plate welding, vibration welding, ultrasonic welding and laser welding, aside from methods that employ fastening parts (bolts, screws, clips and the like) or adhesives. The hot plate welding is a technique in which areas to be welded of molded articles are brought into contact with heated plates so as to be molten, and the molten areas are pressed against each other before cooling and solidifying. However, this technique often forms threads when the areas to be welded are separated from the heated plates, affecting appearance of molded articles. The vibration welding and ultrasonic welding are techniques in which one article is fixed, and the other article is pressed thereon while vibration or ultrasonic wave is applied, so that the areas to be welded are molten and welded by friction energy. However, these techniques often cause dust and thread-like flashes in the welded areas or cause bent flashes or other problems. As a result, molded articles are often degraded in appearance and become unusable depending on shape of the articles.
The laser welding is a method in which one of the articles to be welded is formed of a laser-absorbing material whilst the other is formed of a laser-transmitting material, and they are stacked together and then subjected to irradiation with laser light on the side of the laser-transmitting material. Thus, the laser light which has passed through the transmitting material heats up and melts the surface of the laser-absorbing material, and also melts the laser transmitting material by heat transfer, so that the two resins are welded together (See, Japanese Patent Laid-Open Publication No. 2001-71384). This technique is advantageous in that no threads, dust or flashes are formed, the resulting welded areas have good appearance, and it can be used to weld molded articles of various shapes. Nonetheless, the technique has a limitation in that one of the articles to be welded must be formed of a laser-transmitting molding material
Rubber-reinforced styrenic resins represented by acrylonitrile-butadiene-styrene (ABS) resins, high impact polystyrenes (HIPS) and the like are excellent in mechanical properties, physical properties, electrical properties and the like, and are therefore used in a wide range of fields such as of electrics or electronics, office automation or household electrical appliances, automobiles, and sanitary fittings. However, these resins are low in transmittance of laser light and therefore cannot be used in laser welding unless they are welded with an article made of a laser-transmitting material.